A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element one or more spatial light modulators, each based on a ferroelectric liquid crystal (FLC) material.
A FLC-based spatial light modulator is composed of a layer of a FLC material, preferably a surface-stabilized FLC material, sandwiched between a transparent electrode and a reflective electrode that is segmented into an array of pixel electrodes to define the picture elements (pixels) of the spatial light modulator. The reflective electrode is located on the surface of a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from an input video signal.
The direction of an electric field applied between each pixel electrode and the transparent electrode determines whether or not the corresponding pixel of the spatial light modulator rotates the direction of polarization of light reflected by the pixel. The reflective spatial light modulator is constructed as a quarter-wave plate so that the polarized light reflected by the pixels of the spatial light modulator is either rotated by 90.degree. or not depending on the direction of the electric field applied to each pixel. A polarization analyzer is in the optical path of the light reflected by the spatial light modulator. The polarization analyzer is aligned to either: 1) transmit the polarized light which has rotated and absorb the polarized light which as not been rotated; or 2) transmit the polarized light which as not rotated and to absorb the polarized light which has been rotated. The resulting optical characteristics of each pixel of the spatial light modulator are binary: the light reflected by the pixel either is transmitted through the polarization analyzer (its 1 state) or is absorbed by the polarization analyzer (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
To produce the grey scale required for conventional display devices, the apparent brightness of each pixel is varied by temporally modulating the light transmitted by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the drive signal has been applied to a pixel electrode to cause the pixel to modulate the light passing through it, the DC balance of the pixel must be restored. This is typically done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary drive signal having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal during the illumination period. The illumination period and the balance period collectively constitute a display period. To prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image, the light source illuminating the light valve is modulated, either directly or with a shutter, so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period. However, modulating the light source as just described reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary. This means that a light source of approximately twice the intensity, with a corresponding increase in cost, is necessary to achieve a given display brightness. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
One way this shortcoming can be overcome is the use of a switchable half wave plate also known as a light doubler. In the first of the two states, the 0 state, the switchable half-wave plate leaves the sense of operation of the light valve relative to the direction of the electric field applied to the liquid crystal material of the spatial light modulator unchanged. In the second of the two states, the 1 state, the switchable half-wave plate inverts the sense of operation of the light valve relative to the direction of the electric field applied to the liquid crystal material of the spatial light modulator. The light doubler is typically operated in the 0 state during the illumination period and in the 1 state during the balance period, during which the light source is not modulated (or modulated for only as long as it takes to switch between the 0 state and the 1 state).
Thus, the pixel electrode is driven with a complementary drive signal during the balance period and the DC balance of the pixel is restored. Since the light source is not modulated, light is transmitted by the pixel and its sense of operation is inverted by the switchable half-wave plate in the 1 state. This results in a pixel with the same apparent brightness during a balance period as during a illumination period. The apparent brightness of the pixel operated in conjunction with a switchable half-wave plate is therefore doubled during each display period
FIG. 1 shows part of a conventional display device 5 incorporating a conventional reflective light valve system 10 that includes the reflective spatial light modulator 12 and a switchable half-wave plate 11. Other principal components of the light valve are the polarizer 14, the beam splitter 16 and the analyzer 18. The light valve is illuminated with light from the light source 20, the light from which is concentrated on the polarizer using a reflector 22 and collector optics 24. The light output by the light valve passes to the imaging optics 26 that focus the light to form an image (not shown). The light valve 10, light source 20 and imaging optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
Light generated by the light source 20 enters the light valve 10 by passing through the polarizer 14. The polarizer polarizes the light output from the light source. Alternatively, a polarized light source (not shown) can be used and the need for the polarizer 14 would be eliminated. The beam splitter 16 then reflects a fraction of the polarized light output from the polarizer towards the switchable half-wave plate 11. The beam splitter can additionally or alternatively be a polarizing beam splitter configured to reflect light having a direction of polarization parallel to the direction of polarization of the polarizer 14 towards the switchable half-wave plate 11. The switchable half-wave plate 11 then transmits the light to the spatial light modulator 12.
The spatial light modulator 12 is divided into a two-dimensional array of picture elements (pixels) that define the spatial resolution of the light valve 10. Light reflected from the spatial light modulator is again transmitted by the switchable half-wave plate 11, this time towards the beam splitter 16 which transmits a fraction of the reflected light to the analyzer 18. If the beam splitter is a polarizing beam splitter, however, only light having a direction of polarization orthogonal to the direction of polarization imparted by the polarizer will be transmitted and the need for an independent analyzer may be eliminated.
The direction of electric fields in each pixel of the spatial light modulator 12 and in the switchable half-wave plate 11 determines whether or not the direction of polarization of the light transmitted by the switchable half-wave plate towards the beam splitter is rotated by 90.degree. relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modulator passes through the switchable half-wave plate 11, beam splitter 16, and the analyzer 18 and is then output from the light valve 10 through the imaging optics 26 depending on whether or not its direction of polarization was rotated by approximately 90.degree. by the spatial light modulator and the switchable half-wave plate.
More specifically, the polarizer 14 polarizes the light generated by the light source 20 that passes through the collector optics 24 either directly or after reflecting off reflector 22. The polarization is preferably linear polarization. The beam splitter 16 reflects the polarized light output from the polarizer towards the switchable half-wave plate 11 which transmits the light on to the spatial light modulator 12. The polarized light reflected from the spatial light modulator transmits to the analyzer 18 through the switchable half-wave plate 11 and beam splitter 16. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
For purposes of this description, the terms parallel and orthogonal will be used to describe directions of polarization and directions of maximum transmissivity and maximum reflectivity. When these terms are used within this description, it is understood that they relate to the optical characteristics of the light and of the various components that comprise the light valve and not necessarily to their spatial relationships. For example, when polarized light reflects from a mirror at an angle of incidence of 45.degree., the polarized light is reflected at an angle of 90.degree. relative to the incident light. Even though the incident light and the reflected light are spatially orthogonal to one another, the direction of polarization of the reflected light will be optically unchanged from the direction of polarization of the incident light. Thus, the direction of polarization of the reflected light may be said to be parallel to the direction of polarization of the incident light. In addition, as used herein the term parallel shall include directions that are both parallel and anti-parallel, i.e., having a direction 180.degree. opposed, to the original direction.
The switchable half-wave plate 11 is composed of a first transparent electrode 13 deposited on the surface of a first transparent cover 15, a second transparent electrode 17 deposited on the surface of a second transparent cover 19, and a ferroelectric liquid crystal layer 21 sandwiched between the first and the second transparent electrodes 13, 17. A drive circuit (not shown) applies a drive signal to the one of the first and second transparent electrodes 13, 17 ("driven transparent electrode"). The drive signal has two different voltage levels, and one of two driving schemes is typically used. In the first scheme, the transparent electrode that is not driven is maintained at a fixed potential mid-way between the voltage levels of the drive signal. In the second scheme, the transparent electrode that is not driven has the opposite of the two drive voltage levels of the drive signal applied to it than is applied to the driven transparent electrode. Thus the two different voltage levels of the drive signals are swapped between the first and the second transparent electrode 13, 17. The potential difference between the first and the second transparent electrode establishes an electric field across the liquid crystal layer 21.
The spatial light modulator 12 is composed of a transparent electrode 28 deposited on the surface of a transparent cover 30, a reflective electrode 32 located on the surface of the semiconductor substrate 34, and a ferroelectric liquid crystal layer 36 sandwiched between the transparent electrode 28 and the reflective electrode 32. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially reduced number of pixel electrodes are shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600.times.1200 pixel electrodes. An exemplary pixel electrode is shown at 38. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards the switchable half wave plate 11.
A drive circuit, which may be located in the semiconductor substrate 34, applies a drive signal to the pixel electrode 38 of each pixel of the spatial light modulator 12. An example of such a drive circuit 50 with signal processing electronics and an exemplary pixel is shown in FIG. 2. Regarding this figure and those that follow, it is noted that identical reference numerals are used to designate identical or similar elements throughout the several views, and that elements are not necessarily shown to scale. The drive circuit has external connections to a steady state voltage source with voltage level V.sub.SS, ground, and a video signal 40. The drive circuit applies a drive signal to the pixel electrode 38 of each pixel of the spatial light modulator 12. The drive signal can be switched between two voltage levels, V.sub.SS and ground. The switching of the drive signal supplied to the pixel electrode 38 is controlled by the signal processing electronics 52 though two transistors T.sub.1 and T.sub.2 in response to a portion of the video signal 40. The gates of the two transistors are in electrical communication with the signal processing electronics at nodes P+.sub.1 and P-.sub.1, respectively. Other nodes are provided in the signal processing electronics for providing a drive signal to the remaining pixels (not shown).
Two matched resistors, R1 and R2, connected in series between V.sub.SS and ground, are used to provide a voltage level equal to 1/2V.sub.SS between the resistors. An isolation amplifier A with unity gain has its input connected between the two matched resistors and its output connected to and the transparent electrode 28. The isolation amplifier ensures that the voltage level at the transparent electrode is maintained at 1/2V.sub.SS a fixed voltage level mid-way between the voltage levels of the drive signal. Without the isolation amplifier A, transient currents that occur at the transparent electrode when the drive signal at the pixel electrode 38 switches between V.sub.S and ground could affect the amount of the current drawn through one of the matched resistors and thus alter the voltage level between the resistors.
The potential difference between the pixel electrode and the transparent electrode establishes an electric field across the part of the liquid crystal layer 36 between the pixel and transparent electrodes. The direction of the electric field along with the direction of the electric field of the switchable half-wave plate 11 determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
Since light passes through the reflective spatial light modulator twice, once before and once after reflection by the reflective pixel electrodes, the reflective spatial light modulator 12 is structured as a quarter-wave plate. The thickness of the layer of ferroelectric liquid crystal material in the liquid crystal layer 36 is chosen to provide an optical phase shift of 90.degree. between light polarized parallel to the director of the liquid crystal material and light polarized perpendicular to the director. The liquid crystal material is preferably a Smectic C* surface stabilized ferroelectric liquid crystal material having an angle of 22.5.degree. between its director and the normal to its smectic layers. Reversing the direction of the electric field applied to such a liquid crystal material switches the director of the material through an angle (.phi.) of about 45.degree..
Similarly, light passes through the switchable half-wave plate 11 twice, once before and once after reflection by the reflective spatial light modulator 12. The thickness of the layer of ferroelectric liquid crystal material in the liquid crystal layer 21 is preferably chosen to provide an optical phase shift of equal to half that of the spatial light modulator. In this case, the liquid crystal material is preferably a Smectic C* surface stabilized ferroelectric liquid crystal material having an angle of 11.25.degree. between its director and the normal to its smectic layers. Reversing the direction of the electric field applied to such a liquid crystal material switches the director of the material through an angle (.theta.=.phi./2) of about 22.5.degree..
FIGS. 3A-3E illustrate the actions of the switchable half-wave plate 11 and the spatial light modulator 12 on the direction of polarization of light passing through the light valve 10 at four different points along the optical path of the light valve and in the four possible combinations of the 0 and 1 states of the switchable half-wave plate and each of the spatial light modulators. The points along the optical path are marked A-D in the schematic view of the light valve shown in FIG. 6A. The points are the point A, where the polarized light received at the light input 106 enters the switchable half-wave plate 11 after reflection by the beam splitter 16; the point B, where the light transmitted through the switchable half-wave plate enters the spatial light modulator 12; the point C where the light reflected by the spatial light modulator enters the switchable half-wave plate; and the point marked D where the light transmitted by the switchable half-wave plate enters the beam splitter 16.
In the example shown, in the 0 states of the switchable half-wave plate 11 and the spatial light modulator 12, the director angle ("principle axis") 110 and 114 of these elements are both aligned parallel to the direction of maximum reflectivity of the beam splitter 16, which corresponds to the direction of polarization of the light received at the light input (the first direction). Moreover, the principle axis 110 of the switchable half-wave plate rotates through an angle of 22.5.degree. and the principle axis 114 of the spatial light modulator rotates through an angle of 45.degree. between the 0 state and the 1 state of these elements.
FIG. 3B shows the actions of the switchable half-wave plate 11 and the spatial light modulator 12 when both are in their 0 states. In this state, the principal axes 110 and 114 of the switchable half-wave plate 102 and the spatial light modulator 12, respectively, are both parallel to the direction of maximum reflectivity of the beam splitter 16. Consequently, the direction of polarization 112 of the light received at the light input 106 and reflected by the beam splitter is parallel to the principal axis 110 of the switchable half-wave plate in its 0 state, as shown at A. The switchable half-wave plate therefore transmits the light received at the light input without changing the direction of polarization of this light.
The direction of polarization 116 of the light transmitted by the switchable half-wave plate 11 is parallel to the principal axis 114 of the spatial light modulator 12 in its 0 state, as shown at B. The spatial light modulator therefore reflects the light transmitted by the switchable half-wave plate without changing the direction of polarization of this light.
The direction of polarization 118 of the light reflected by the spatial light modulator 12 is parallel to the principal axis 110 of the switchable half-wave plate 11 in its 0 state, as shown at C. The switchable half-wave plate therefore transmits the light reflected by the spatial light modulator without changing the direction of polarization of this light.
The direction of polarization 120 of the light transmitted by the switchable half-wave plate 11 is orthogonal to the direction of maximum transmissivity 122 of the beam splitter 16, as shown at D. The beam splitter reflects the light transmitted by the switchable half-wave plate away from the light output 108, so that the pixel appears dark when viewed from the light output.
FIG. 3C shows the actions of the switchable half-wave plate 11 and the spatial light modulator 12 when the switchable half-wave plate is in its 0 state and the spatial light modulator is in its 1 state. In this state, the principal axis 110 of the switchable half-wave plate 11 is parallel to, and the principal axis of the spatial light modulator 12 is at an angle of .phi.=45.degree. to the direction of maximum reflectivity of the beam splitter 16. Consequently, the direction of polarization 112 of the light received at the light input 106 and reflected by the beam splitter is parallel to the principal axis 110 of the switchable half-wave plate in its 0 state, as shown at A. The switchable half-wave plate therefore transmits the light received at the light input without changing the direction of polarization of this light.
The direction of polarization 116 of the light transmitted by the switchable half-wave plate 11 is at an angle of 45.degree. to the principal axis 114 of the spatial light modulator 12 in its 1 state, as shown at B. The spatial light modulator therefore rotates the direction of polarization of the light transmitted by the switchable half-wave plate through 90.degree. when it reflects this light. The direction of polarization of the light at B after reflection is indicated at 124.
The direction of polarization 118 of the light reflected by the spatial light modulator 12 is at an angle of 90.degree. to the principal axis 110 of the switchable half-wave plate 11 in its 0 state, as shown at C. The switchable half-wave plate therefore leaves the direction of polarization of the light reflected by the spatial light modulator unchanged when it transmits this light.
The direction of polarization 120 of the light transmitted by the switchable half-wave plate 11 is parallel to the direction of maximum transmissivity 122 of the beam splitter 16, as shown at D. The beam splitter therefore transmits the light transmitted by the switchable half-wave plate to the light output 108, and the pixel appears bright when viewed from the light output.
FIG. 3D shows the actions of the switchable half-wave plate 11 and the spatial light modulator 12 when the switchable half-wave plate is in its 1 state and the spatial light modulator is in its 0 state. In this state, the principal axis 110 of the switchable half-wave plate 11 is at an angle of .theta.=22.5.degree. to, and the principal axis of the spatial light modulator 12 is parallel to, the direction of maximum reflectivity of the beam splitter 16. Consequently, the direction of polarization 112 of the light received at the light input 106 and reflected by the beam splitter is at an angle of 22.5.degree. to the principal axis 110 of the switchable half-wave plate in its 1 state, as shown at A. The switchable half-wave plate therefore rotates the direction of polarization of the light received at the light input through 45.degree. when it transmits this light.
The direction of polarization 116 of the light transmitted by the switchable half-wave plate 11 is aligned at an angle of 45.degree. to the principal axis 114 of the spatial light modulator 12 in its 0 state, as shown at B. The spatial light modulator therefore rotates the direction of polarization of the light transmitted by the switchable half-wave plate through 90.degree. when it reflects this light. The direction of polarization of the light at B after reflection is indicated at 124.
The direction of polarization 118 of the light reflected by the spatial light modulator 12 is at an angle of 67.5.degree. to the principal axis 110 of the switchable half-wave plate 11 in its 1 state, as shown at C. The direction of polarization shown at 118 is the mirror image of the direction of polarization shown at 124 because the view in C is in the opposite direction to the view in B. The switchable half-wave plate therefore rotates the direction of polarization of the light reflected by the spatial light modulator though 135.degree. when it transmits this light.
The direction of polarization 120 of the light transmitted by the switchable half-wave plate 11 is parallel to the direction of maximum transmissivity 122 of the beam splitter 16, as shown at D. The beam splitter therefore transmits the light transmitted by the switchable half-wave plate to the light output 108, and the pixel appears bright when viewed from the light output.
FIG. 3E shows the actions of the switchable half-wave plate 11 and the spatial light modulator 12 when both are in their 1 states. In this state, the principal axis of the switchable half-wave plate 11 is at an angle of .theta.=22.5.degree. to, and the principal axis of the spatial light modulator 12 is at an angle of .phi.=45.degree. to the direction of maximum reflectivity of the beam splitter 16. Consequently, the direction of polarization 112 of the light received at the light input 106 and reflected by the beam splitter 16 is at an angle of 22.5.degree. to the principal axis 110 of the switchable half-wave plate in its 1 state, as shown at A. The switchable half-wave plate therefore rotates the direction of polarization of the light received at the light input through 45.degree. when it transmits this light.
The direction of polarization 116 of the light transmitted by the switchable half-wave plate 11 is parallel to the principal axis 114 of the spatial light modulator 12 in its 1 state, as shown at B. The spatial light modulator therefore reflects the light transmitted by the switchable half-wave plate without changing the direction of polarization of this light. The direction of polarization of the light at B after reflection is indicated at 124, which coincides in direction with 116.
The direction of polarization 118 of the light reflected by the spatial light modulator 12 is at an angle of 22.5.degree. to the principal axis 110 of the switchable half-wave plate 11 in its 1 state, as shown at C. The direction of polarization shown at 118 is the mirror image of the direction of polarization shown at 124 because the view in C is in the opposite direction to the view in B. The switchable half-wave plate therefore rotates the direction of polarization of the light reflected by the spatial light modulator though 45.degree. when it transmits this light.
The direction of polarization 120 of the light transmitted by the switchable half-wave plate 11 is orthogonal to the direction of maximum transmissivity 122 of the beam splitter 16, as shown at D. The beam splitter therefore reflects the light transmitted by the switchable half-wave plate away from the light output 108, so that the pixel appears dark when viewed from the light output.
It can be seen by comparing FIGS. 3B and 3D that, when a spatial light modulator 12 is in its 0 state, the direction of polarization of the light impinging on the beam splitter 16 after reflection from the spatial light modulator is orthogonal to (pixel dark) and parallel to (pixel bright) the direction of maximum transmissivity 122 when the switchable half-wave plate is in its 0 state and in its 1 state, respectively. Similarly, it can be seen by comparing FIGS. 3C and 3E that, when a spatial light modulator is in its 1 state, the direction of polarization of the light impinging on the beam splitter after reflection from the spatial light modulator is parallel to (pixel bright) and orthogonal to (pixel dark) the direction of maximum transmissivity when the switchable half-wave plate is in its 0 state and in its 1 state, respectively. The duration of the 1 state of the spatial light modulator during the illumination period is the same as the duration of the 0 state during the following balance period. However, changing the state of the switchable half-wave plate between the illumination period and the balance period inverts the sense of the light valve 10 relative to the direction of the electric field applied to the liquid crystal material of the spatial light modulator 12. Consequently, the direction of polarization of the light incident on the beam splitter is at the same angle relative to the direction of maximum transmissivity during the same temporal portion of both the illumination period and the balance period.
In a miniature, wearable display, the imaging optics 26 are composed of an eyepiece that receives the light reflected by the reflective electrode 32 and forms a virtual image at a predetermined distance in front of the user (not shown). In a cathode-ray tube replacement or in a projection display, the imaging optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
Since the direction of maximum transmission of the analyzer 18 is orthogonal to the direction of polarization defined by the polarizer 14, light whose direction of polarization has been rotated through 90.degree. by a pixel of the spatial light modulator 12 in combination with the switchable half-wave plate 11 will pass through the analyzer and be output from the light valve 10 whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to the imaging optics 26 light whose direction of polarization has been rotated by pixels of the spatial light modulator in combination with the switchable half-wave plate. The pixels of the spatial light modulator will appear bright or dark depending on the direction of the electric field applied to each pixel and the direction of the electric field in the switchable half-wave plate. When a pixel appears bright, it will be said to be in its 1 output state, and when the pixel appears dark, it will be said to be in its 0 output state.
The direction of maximum transmission of the analyzer 18 can alternatively be arranged parallel to that of the polarizer 14, and a non-polarizing beam splitter can be used as the beam splitter 16. In this case, the spatial light modulator 12 operates in the opposite sense to that just described. Another possible configuration that eliminates the beam splitter 16 altogether is shown in FIG. 4. In this configuration, the light source 20, collector optics 24, and polarizer 14 are arranged linearly, but at an angle to the reflective spatial light modulator 12. Thus, light reflects from the spatial light modulator at an angle opposite the angle of incidence towards the analyzer 18 and imaging optics 26. The switchable half-wave plate 11 is positioned to both transmit light to the spatial light modulator 12 from the polarizer 14 and to transmit light reflected from the spatial light modulator towards the analyzer 18.
To produce the grey scale required by a display device notwithstanding the binary optical characteristics of the pixels of the light valve 10, the apparent brightness of each pixel is varied by temporally modulating the light reflected by the pixel, as described above. The drive circuit 50 like that shown in FIG. 2, for each pixel of the spatial light modulator determines the duration of the 1 state of the pixel in response to a portion of the input video signal 40 corresponding to the location of the pixel in the spatial light modulator.
FIGS. 5A-5F illustrate the operation the light valve 10 shown in FIG. 1 utilizing an exemplary pixel controlled by the pixel electrode 38, in spatial light modulators 12 during three exemplary frames of the input video signal 40. Each of the display periods corresponds to one frame of the input video signal and is composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations, as shown in FIG. 5A.
FIG. 5B shows the drive signal applied to the pixel electrode 38. During the balance period, the level of each drive signal is 0 and V.sub.SS for times equal to the times that it was at V.sub.SS and 0, respectively, during the illumination period, so that the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
FIG. 5C shows the state of the switchable half-wave plate 11. In the 0 state, the direction of the principal axis of the switchable half-wave plate is aligned parallel to the direction of polarization of the polarizer 14. This corresponds to the direction of maximum reflectivity of the beam splitter 16. As a result, the direction of polarization of the light generated by the light source remains unchanged after passing though the switchable half-wave plate in its 0 state. Thus, when the light that has passed through the switchable half-wave plate in its 0 state impinges on the spatial light modulator 12, its direction of polarization is parallel to the principal axis of any of the spatial light modulators that are in the 0 state, and is at an angle of .phi.=2.theta. to the principal axis of any of the spatial light modulators that are in the 1 state.
In the 1 state of the switchable half-wave plate 11, the direction of the principal axis is at the non-zero angle .theta. to the direction of polarization of the incident light. The value of .theta. is discussed above. As a result, the direction of polarization of the light from the light source 20 and polarized by polarizer 14 is rotated through an angle of 2.theta. by passing though the switchable half-wave plate in this state. Thus, when the light that has passed through the switchable half-wave plate in its 1 state impinges on the spatial light modulator 12, its direction of polarization is at an angle of .phi.=2.theta. to the principal axis of the spatial light modulator in the 0 state, and is parallel to the principal axis of the spatial light modulator in the 1 state.
FIG. 5D shows the combined effect of the pixel electrode 38 and the switchable half-wave plate 11 on the direction of polarization of the light impinging on the beam splitter 16 after reflecting from the pixel electrode 38 and passing through the switchable half-wave plate 11 a second time. The direction of polarization is indicated by the absolute value of the angle .alpha. between direction of polarization of the light impinging on the beam splitter and the direction of maximum transmissivity of the beam splitter. The beam splitter transmits light having an angle .alpha. close to zero and reflects light having an angle .alpha. close to 90.degree.. In this Figure, the light source 20 is unmodulated to show the timing of the changes in the direction of polarization of the light impinging on the beam splitter.
FIG. 5E shows the modulation of the light generated by the light source 20. The light source is ON throughout most the illumination period and most of the balance period of each display period, and is OFF only during the brief periods during which the switchable half-wave plate 11 is changing state.
FIG. 5F show the light output from the light valve 10 after having been reflected by the pixel electrode 38. The durations of the temporal portions of both the illumination period and the balance period of each display period during which light is output are the same. Since the light source 20 is modulated as shown in FIG. 5F, the light valve is not illuminated during the switching transients of the switchable half-wave plate 11. The remaining pixels operate similarly.
The light valve 10 shown in FIG. 1 can also be adapted to provide a colored light output to the imaging optics 26. One way that this can be done is by replacing the "white" light source 20 with three colored light sources such as a red, blue and green LEDs (not shown), each illuminating the spatial light modulator 12 sequentially. This would require a balance period after each sequential illumination period. Another way that a colored light output can be provided is by replacing the single reflective spatial light modulator 12 shown in FIG. 1 with three reflective spatial light modulators and a color separator for separating the light into three component colors.
An example of one such color configuration is depicted in FIG. 6. In this figure, the color separator is a series of three dichroic plates 42, 43, 44, each having an associated reflective spatial light modulator 12. Each of the dichroic plates is configured to reflect light in a band of wavelengths (colorband) particular to that dichroic plate and to pass the remaining wavelengths of light. Thus, if the light source 20 is a "white" light, emitting visible light across the entire visible color spectrum, a particular portion of the color spectrum may be reflected by each dichroic plate its associated reflective spatial light modulator simultaneously. This eliminates the need for sequential illumination and improves the perceived brightness of the color pixels passing through the analyzer.
For example, the dichroic plate 42 nearest the beam splitter 16 might reflect red-colored light toward its associated spatial light modulator 12 while the center dichroic plate 43 reflects green-colored light toward its associated spatial light modulator and the dichroic plate remote from the beam splitter 44 reflects blue-colored light towards its spatial light modulator. When the light source 20 is ON, as shown if FIG. 6, the colored light reflected by the dichroic plates passes to each of the three reflective spatial light modulators 12. Each of the three reflective spatial light modulators is capable of reflecting pixels of the colored light back at its associated dichroic plate in a manner consistent with the above description of the operation of the spatial light modulator shown in FIG. 1.
The majority of the colored light reflected by each of the spatial light modulators 12 will be reflected by its associated dichroic plate toward the analyzer 18 since the light reflected by each spatial light modulator will retain the characteristic wavelengths of light originally reflected by its respective dichroic plate. When the combined colored light from each of the three reflective spatial light modulators 12 passes through the analyzer, a full color image can be formed by the imaging optics 26.
Whether the light valve includes a single spatial light modulator 12 or three, it is important that a high contrast ratio between the 1 output state and the 0 output state be maintained. The ratio of the intensity of light transmitted in the 1 state to the intensity of light transmitted in the 0 state is known as the "contrast ratio" or simply as "contrast." A contrast ratio of at least 100:1 is required for excellent image quality and is usually found in CRT based displays. The precise alignment of the direction of polarization of light reflected from each pixel and passing through the switchable half-wave plate either parallel to or orthogonal to the direction of maximum transmissivity of the analyzer 18 (or a polarizing beam splitter) is of critical importance in order to maintain the high contrast ratio. Even slight misalignment will cause the a portion of the light which should be transmitted by the analyzer 18 in the 1 state to be absorbed (or reflected) and a portion of the light which should be absorbed (or reflected) by the analyzer (or polarizing beam splitter) in the 0 state to be transmitted. Light transmitted through the analyzer in the 0 state is known as "dark state luminance" and significantly influences image quality and quickly drives the contrast ratio down.
The precise alignment of the direction of polarization reflected by the pixels and passing through the switchable half-wave plate 11 is affected by a number of mechanical tolerances encountered during the assembly of the light valve 10. Tolerances in the mechanical alignment of the polarizer 14, the spatial light modulator 12, the switchable half-wave plate 11, and the analyzer 18 are typically plus or minus one-half degree and contribute to misalignment. In addition, during assembly of the spatial light modulator 12, the angle at which the normal to the smectic layers of the ferroelectric liquid crystal material 36 is aligned relative to the substrate 34 (pre-alignment angle) has a tolerance that is also typically plus or minus one-half degree that may contribute to misalignment. Similar misalignment may occur during assembly of the switchable half-wave plate. Once the light valve 10 is assembled, these tolerances result in a permanent fixed offset.
In addition to the permanent fixed offset, however, the direction of polarization of light reflected from the pixels and transmitted through the switchable half-wave plate is affected by variations in the tilt angle of the ferroelectric liquid crystal material with temperature change. Tilt angle is the angle that the director of the ferroelectric liquid crystal material switches through when the direction of the electric field across the ferroelectric liquid crystal material is reversed. The tilt angle is symmetric around the pre-alignment angle. Thus, if the director of the liquid crystal material at a particular temperature forms an angle of 22.5.degree. with the normal to its smectic layers when exposed to an electric field having a forward direction, the director switches through a tilt angle of 45.degree. when the direction of the electric field reverses. Tilt angle usually changes by between one and two degrees during a change from room temperature to operating temperature of the spatial light modulator. Operating temperature for a spatial light modulator is typically 60.degree. C. with the increase in temperature coming from heat generated by the operation of the drive circuits 50 and the absorption of energy from the light illuminating the spatial light modulator.
FIGS. 7A and 7B show how variations in tilt angle with temperature affect operation of the light valve. The switchable half-wave plate is not shown in these figures for clarity. FIG. 7A depicts an analyzer 18 and a ferroelectric liquid-crystal-based spatial light modulator 12 at room temperature, such as when a light valve is first turned on. FIG. 7B shows the same analyzer and spatial light modulator at normal operating temperature. The direction of maximum transmissivity of the analyzer is indicated in both figures by line 54 with closed arrow heads, while the orthogonal direction of minimum transmissivity is shown by the line 56 with open arrow heads. The direction of polarization of light reflected by pixels having an electric field in the forward direction is depicted in both figures by line 58 with closed arrow heads. The direction of polarization of light reflected by pixels having an electric field in the reverse direction is depicted in both figures by line 60 with closed arrow heads.
In FIG. 7A, the angle between the direction of polarization of the light reflected from pixels having a forward electric field and pixels having a reverse electric field is indicated as a room temperature angle (RTA) greater than 90.degree.. While RTA would actually be only slightly greater than 90.degree., its magnitude has been exaggerated in the figure for clarity. The slight misalignment between the direction of polarization of light reflected from the spatial light modulator 58, 60 and the direction of maximum or minimum transmissivity of the analyzer 54, 56 reduces the contrast ratio of the light valve. The misalignment between the direction of polarization of light having a direction 60 and the direction of minimum transmissivity 56 has a substantially greater effect on the contrast ratio than does the misalignment between the direction of polarization of light having a direction 58 and the direction of maximum transmissivity 54.
In FIG. 7B, the angle between the direction of polarization of the light reflected from pixels having a forward electric field and pixels having a reverse electric field is indicated as an operating temperature angle (OTA) less than 90.degree.. While OTA would actually be only slightly less than 90.degree., its magnitude has been exaggerated in the figure for clarity. Again, the slight misalignment between the direction of polarization of light reflected from the spatial light modulator 58, 60 and the direction of maximum or minimum transmissivity of the analyzer 54, 56 reduce the contrast ratio of the light valve. In this case, the performance of the light valve at neither room temperature or operating temperature is optimal, but a compromise exists so the performance at room temperature is not noticeably worse than it is at room temperature.
The problem of change in tilt angle change and of misalignment is amplified when a switchable half-wave plate is used because the optical path encounters this misalignment twice, once on the way to the special light modulator and once after being reflected by the spatial light modulator. Thus, a difference (.beta.) between the angle .theta. through which the principal axis of the switchable half-wave plate 11 switches and one-half of the actual angle .phi. through which the principal axis of the spatial light modulator 12 switches ultimately causes an offset in the alignment angle equal to twice the difference (2.beta.). This reduces the contrast ratio of the light valve 10 more than other misalignments in the light valve system. For example, a difference between .theta. and .phi./2 of less than about .+-.3.5.degree. is required to provide a contrast of 20 dB (10:1), and a difference of less than about .+-.1.5.degree. is required to provide a contrast of 40 dB (100:1).
An example of this is depicted in FIGS. 8A-8D which are similar to FIGS. 3B-3E but with an indicated offset equal to the difference (.beta.). Since the difference (.beta.) is subtracted from the tilt angle in our example, a 1/2.beta. change is shown for each of the director angles. The 2.beta. error which results is clearly indicated in column D for each possible combination of electric field directions.
One item that tends to introduce this difference (.beta.) between the angle .theta. through which the principal axis of the switchable half-wave plate 11 switches and one-half of the actual angle .phi. through which the principal axis of the spatial light modulator 12 switches, is the change of tilt angle with temperature change. The spatial light modulator 12 and the switchable half wave plate 11 tend to heat at different rates and the spatial light modulator also tends to have a higher normal operating temperature. This causes a changes in the tilt angles at different rates for the spatial light modulator and the switchable half-wave plate. Manufacturers have attempted to correct this problem by installing heater circuits in the switchable half-wave plate, but temperature of the switchable half wave plate must be maintained within .+-.2.degree. C. of the spatial light modulator in order to maintain a good contrast ratio. This temperature tolerance is very difficult to achieve at a product level.
In addition, heaters do not offer the dynamic minute to minute adjustment of contrast control that may be appropriate, especially during warmup, nor do they offer a user the ability to adjust contrast. Consequently, what is needed is a light valve system with instantaneous correction of tilt angle in the switchable half-wave plate and the ability for a user to change contrast settings.